Curve fitting of PSD?

Hello, I need help for the curve fitting. I am using the following command but there is some problem with it.

NFFT=2^nextpow2(L);

Y1=fft(y-mean(y))/L; % Y1 is FFT

fc=Fs/2*linspace(0,1,NFFT/2+1);

Y2=2*abs(Y1(1:NFFT/2+1));%absolute value of FFT

Nc=(Y2).^2; % Nc is PSD

plot(fc,Nc);

g1=fittype('a1-x*p1', 'coef',{'a1','p1'});

options=fitoptions('Method','nonlinearleastsquares','lower',[-25 0], 'upper',[-5 4]);

options.startpoint=[-12,1];

options.TolFun=1e-12;

loglog(fc(1:length(fc)), Nc(1:NFFT/2+1),'b');

curvec= FIT(log10(fc(1:length(fc))),log10(Nc(1:NFFT/2+1)),g1);

=------------------------------- could you please tell me what is wrong with the command?

Thanks